Gully erosion in southern Nigeria: an integrated geological and electrical resistivity study. – C. N. Ehirim and J. O. Ebeniro
The study area lies in the rainforest belt of sourtheastern Nigeria, an area of about 15, 600Km2, devastated by acute and chronic gully erosion. It is underlain by a sequence of Creaceous to Recent sedimentary rocks, which are dominantly shally, tending formations towards the top. The region is characterized by steep slopes, complex geological and tectonic settings and bordered to the east and west by uplifted Precambrian crystalline rocks of the basement. We investigated whether drainage density and coefficient of anisotropy do actually affect gully development. Drainage linears were analyzed statistically and expressed as drainage densities Dd, while azimuthal resistivity soundings were used to develop anisotropy figures from which the coefficients of anisotropy ( ) were determined. The drainage density and coefficient of anisotropy varies from 0.12 to 0.64 (km/km2) and from 1.06 to 1.50, respectively. Each of these data set was grouped into three frequency classes and integrated with ground-based information to determine gully frequency in each class interval. Gully frequency was found to be maximum in high drainage density and coefficient of anisotropy classes, which suggested that areas of high surface flows and underlying structure were more susceptible to gully related activities. The results revealed a direct relationship between the occurrence of gully erosion and these terrain parameters. It thus followed that gully erosion in the belt is controlled by structures (joints and faults) and lithology. The Ajali sandstones and Nanka Sands suffer severe gully erosion while the other lithologic units suffer from slight to moderate gully erosion.